Xserve IP failover…

One of the main attractions with the Xserve which is making my company lean towards buying one (two) is it's ability for a backup Xserve to monitor the primary one and assume it's IP if the worst should happen. My question is can you get a backup Xserve to continually mirror the primary in real time so that the files stored on the two are exactly the same? If not how often would the backup refresh? Or do you need to do this manually with retrospect? Any help would be appreciated.